Create a config variable representing the time interval whereafter
graph data gets pruned. This value should be used to limit lookup
time frames.

If the latest node details have already been seen by a client, we
still need to store them for correctly detecting reminder necessity
in the future.

We want to ignore any node announcements that have already been
pruned. To do so, we extract all the node IDs from the network graph,
and use those to filter our queries.

We will need to determine whether or not a snapshot should include
reminders for both channel and node update messages. To prepare for
that, we extract the decision logic into its own method.

With the addition of reminders, we may encounter scenarios where
either a bit flip may suffice, instructing the client to look up its
latest data, or we may need to serialize all announcement details
a new if the client may have already purged the old data.

To better distinguish between these scenarios, we introduce a
serialization strategy enum that allows serializing either the full
announcement, just the mutations, or serve solely as a reminder and
serialize nothing at all.

This covers the following part of our serialization logic:

If the pre-sync update was more than 6 days ago, serialize in full.